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<br />'~., <br />'\ <br />''', . j-- <br /> <br />analyses classified soil samples from the program as "mostly eL's <br />with a plasticity index of around 20". <br /> <br />" <br />... <br />-, <br /> <br />Beddinq Attitude <br /> <br />A 1937 Preliminary Geology Report describes the bedrock <br />bedding as being nearly horizonta~. Bedrock outcrops are scarce <br />and surface observations are possible only on scattered sandstone <br />outcrops. Test pits and drill holes used in the 1936 <br />investigations enabled the determination of bedding attitude. <br /> <br />Permeability of Rock Foundation <br /> <br />Testing for permeability in the 1937 program resulted in a <br />number of these tests having moderate drill hole water losses. <br />The drill hole logs contain numerous comments that the water <br />losses were probably due to flow around the casing and that the <br />rock is less pervious than the tests indicate. However, the <br />geologists were concerned about the perviousness of the rock <br />nearest the surface due to its fractured and desiccated nature <br />and were concerned that a cutoff extend into the rock if a dam <br />were to be constructed. <br /> <br />Seismotectonics <br /> <br />The damsite lies near the boundary between the Colorado <br />Plateau and Middle Rocky Mountain physiographic and tectonic <br />provinces (Kirkham and Rogers, 1981). This region is <br />characterized by few documented late Cenozoic faults. However, <br />comprehensive surveys have not been conducted to catalog recent <br />faulting in this area. The closest documented Quaternary faults <br />to the site are the Ridgway fault (MCE: Ml 6.5, distance = 115kID) <br />and the Sawatch Fault (MCE: MS 7, distance = 120 kID). Given <br />these magnitudes and distances, the closest preliminary seismic <br />source to the site will be a "random" earthquake unrelated to <br />mapped surface faults. <br /> <br />The proposed Ridges Basin Dam (Animas-La Plata project) is <br />also located along the eastern boundary of the Colorado Plateau. <br />Recent studies for that site (Piety and Vetter, 1992) defined a <br />maximum credible random event of Mt,.6. 5 at an epicentral distance <br />of 14 kID for an annual probability of 2x10-5. The Ridges Basin <br />site is over 200 kID to the south of the proposed Owens Creek dam <br />site. As a result, questions regarding the applicability of the <br />Ridges Basin results may be raised. Additional seismotectonic <br />evaluations are currently being conducted for Ruedi and Rifle Gap <br />Dams as well as regional studies for Northern and Central <br />Colorado. The results of these studies will be available for <br />preliminary design and should provide much of the information <br />required for a comprehensive seismotectonic evaluation of the <br />Owens Creek site. However, some questions regarding potential <br /> <br />4 <br />